This happens to me pretty often. For example, right now I have the debugger stopped at a breakpoint in a method . . . and it isn\'t displaying any variable values at all.
The most common reason for this is that you're trying to debug code compiled with optimisation enabled and/or no debug symbols. Typically this will be because you're trying to debug a Release build rather than a Debug build but it can also happen with Debug builds if you've made inappropriate changes to the Debug build settings.
Another less common possibility is that you've hosed the stack.